Optical recording/reproducing apparatus for optical disks with various disk substrate thicknesses

ABSTRACT

An optical disc apparatus for recording, reproducing or erasing an information signal by converging a light flux onto a recording layer through a transparent substrate. The apparatus includes one or a plurality of optical heads having a plurality of objective lenses whose aberrations have respectively been corrected for a plurality of disc substrates of different thicknesses, a cartridge for enclosing the optical disc, a discrimination hole which is formed on the cartridge, and a sensor for detecting the opening/closing state of the discrimination hole and for generating a discrimination signal. In accordance with the result of the discrimination as to the thickness of the loaded optical disc, the objective lens, in which the occurrence of the aberration is smallest, is used, so that the information signal can preferably be recorded, reproduced or erased onto/from the optical discs having different substrate thicknesses. Instead of an optical head having objective lenses, an optical head having a waveguide and a plurality of converging grating couplers whose aberrations have respectively been corrected for a plurality of disc substrates of different thicknesses is provided to achieve the same object.An optical recording/reproducing apparatus for recording, reproducing or erasing an information signal onto/from an optical disc having at least a transparent substrate and an information layer by converging a light flux on the information layer through the transparent substrate. The apparatus includes optical converging devices, with different numerical apertures, focal distances or working distances, such as objective lenses or grating lenses, for performing aberration correction over a plurality of transparent substrates of different thicknesses of optical discs and a device for discriminating the type of optical disc based on the thicknesses of the transparent substrates. One of the optical converging devices that generates the least aberration is used according to a result of the discrimination of the thickness of the optical disk loaded in the apparatus to cause the information signal to be suitably recorded, reproduced or erased onto/from the optical discs having the different substrate thicknesses.

This is a continuation of reissue application Ser. No. 08/396,981 filedMar. 1, 1995 now U.S. Pat. No. RE36,445 which is a reissue of U.S. Pat.No. 5,235,581.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical disc apparatus which can record,reproduce, or erase information signals onto/from both of an opticaldisc having a recording density similar to that of a conventional CD(compact disc) and an optical disc having a recording density higherthan the above recording density.

2. Description of the Prior Art

In recent years, in addition to an optical disc apparatus only forreproduction such as a CD player or the like, an optical disc apparatuswhich can record and reproduce an information signal is actively beingdeveloped.

Ordinarily, the recording and reproduction of an information signalonto/from an optical disc are executed by converging a beam which isradiated from a semiconductor laser or the like onto a recording layerof the optical disc by a lens. The recording layer here denotes a pitlayer in the case of a CD and is a layer in which a deformation, achange in optical constant, a formation of a magnetic domain, or thelike is performed by a converged laser beam in the case of a recordableoptical disc. To raise a recording density of the optical disc, it isnecessary to reduce a spot diameter D of the converged beam. There isthe following relation among the spot diameter D, a numerical apertureNA of the lens, and a wavelength λ of the laser beam. $\begin{matrix}{D \propto \frac{\lambda}{NA}} & (1)\end{matrix}$

The above equation (1) denotes that the beam spot diameter D decreasesby using a lens of a large NA. That is, by increasing NA, the highdensity recording can be executed.

When NA of the lens increases, however, an aberration of the convergedbeam due to an inclination error of the disc called a tilt increases.Particularly, a coma aberration increases. There is the followingrelation among a wave front aberration W_(c) of the coma, a tilt angleα, and NA when using a thickness d and a refractive index n of the discsubstrate. $\begin{matrix}{W_{c} \propto {\frac{n^{2} - 1}{2n^{3}} \cdot d \cdot \alpha \cdot ({NA})^{3}}} & (2)\end{matrix}$

The above equation (2) denotes that in the case of using a lens of NAwhich is larger than that of the conventional lens, even if a tilt angleis identical, the coma aberration increases. It will be understood fromthe equation (2), however, that there is an effect to suppress the comaaberration by setting the thickness d of the disc substrate to be thin.In the optical disc for the high density recording, therefore, it ispreferable that the thickness of the disk substrate is thinner than thatof the conventional optical disc, so that an optical head using anobjective lens corresponding to the thin disc substrate is needed.

On the other hand, even in the optical disc apparatus corresponding tothe high density recording, it is preferable that the conventionaloptical disc of a thick substrate can be also reproduced so that a greatamount of conventional software resources can be utilized.

However, the optical head which has been designed for a thin substratecannot be used for an optical disc of a thick substrate. The reasonswill now be described hereinbelow. The objective lens for an opticaldisc has been designed so as to set off a spherical aberration whichoccurs when the converged beam passes in the disc substrate. Since suchan aberration is corrected in accordance with the thickness of the discsubstrate, the aberration correction is not accurately performed for theconverged beam which passes through the disc substrate having athickness different from the design value. The above point will now beexplained with reference to the drawing. FIGS. 18A and 18B are schematicside elevational views for explaining a situation of the occurrence ofthe aberration due to the disc substrates having different thicknesses.FIG. 18A is a diagram in the case of using an objective lens which hasbeen designed for a thin disc substrate and shows traces of lights in astate in which a beam has been converged through the disc substratehaving the same thickness as the design value. In the diagram, a brokenline indicates the surface of a recording layer and all of the lightsemitted from the objective lens are converged to one point 0 on therecording layer surface. FIG. 18B is a diagram in the case of using anobjective lens which has been designed for the same thin disc substrateas that of FIG. 18A and shows traces of lights in a state in which thebeam has been converged through the disc substrate having a thicknesswhich is thicker than the design value. In FIG. 18B, the lights emittedfrom the outermost peripheral portion of the objective lens areconverged to a point O′ on the recording layer surface. However, thelight locating near the optical axis is converged at the front side.Such a phenomenon is called a spherical aberration. When such anaberration occurs the objective lens cannot converge the light beamuntil what is called a diffraction limit. Therefore, in the case ofusing the objective lens whose aberration has been corrected for a thindisc substrate, an information signal cannot be recorded, reproduced, orerased onto/from an optical disc having a thick disc substrate.Similarly, in the case of using the objective lens whose aberration hasbeen corrected for a thick disc substrate, an information signal cannotbe recorded, reproduced, or erased onto/from an optical disc having athin disc substrate.

SUMMARY OF THE INVENTION

In consideration of the above drawbacks, it is an object of theinvention to provide an optical disc apparatus which can record,reproduce, or erase information signals onto/from a plurality of opticaldiscs in which thicknesses of disc substrates are different.

To accomplish the above object, according to the invention, there isprovided an optical disc apparatus for recording, reproducing, orerasing information signals onto/from an optical disc by converginglight fluxes onto a recording layer through a transparent discsubstrate, comprising: N converging means whose aberrations have beencorrected for N (N≧2) disc substrates having different thicknesses,respectively; disc discriminating means for discriminating the thicknessof the disc substrate of the optical disc which has been loaded and forgenerating a discrimination signal according to the result of thediscrimination; and control means for selecting one of the convergingmeans in which the occurrence of the aberration due to the discsubstrate is smallest in accordance with the discrimination signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a constructional diagram of an optical disc apparatusaccording to the first embodiment of the invention;

FIG. 2 is a plan view showing an arrangement of a main section of theoptical disc apparatus in the first embodiment;

FIGS. 3A and 3B are perspective views of cartridges of optical discs inthe first embodiment;

FIGS. 4A and 4B are schematic diagrams showing cross sections of theoptical discs and states of convergences by objective lenses in thefirst embodiment;

FIG. 5 is a constructional diagram of an optical disc apparatusaccording to the second embodiment of the invention;

FIG. 6 is a detailed constructional diagram of optical heads in thesecond embodiment;

FIG. 7 is a constructional diagram of an optical disc apparatusaccording to the third embodiment of the invention;

FIG. 8 is a detailed constructional diagram of an optical head in thethird embodiment;

FIGS. 9A and 9B are a detailed constructional diagram of an optical headof an optical disc apparatus according to the fourth embodiment of theinvention;

FIG. 10 is a cross sectional view showing optical discs whose substratethicknesses are different and converging states by convergence gratingcouplers which have been designed in correspondence to the optical discsaccording to the fifth embodiment, respectively;

FIG. 11 is a schematic perspective view showing a construction of anoptical head of an optical disc apparatus in the fifth embodiment;

FIG. 12 is a schematic perspective view showing a construction of anoptical head of an optical disc apparatus according to the sixthembodiment of the invention;

FIG. 13 is a schematic perspective view showing a construction of anoptical head of an optical disc apparatus according to the seventhembodiment of the invention;

FIG. 14 is a block diagram showing a construction of the optical discapparatus in the seventh embodiment;

FIG. 15 is a block diagram showing a construction of an optical discapparatus according to the eighth embodiment of the invention;

FIG. 16 is a schematic enlarged perspective view showing a convergencegrating coupler of an optical head, an SAW transducer, and a portion inwhich surface acoustic waves have been formed in the eighth embodiment;

FIG. 17 is a characteristic diagram for explaining the principle of thetracking control in the eighth embodiment; and

FIGS. 18A and 18B are schematic side elevational views showing occurringsituations of aberrations due to disc substrates having differentthicknesses.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention will be described hereinbelow on theassumption that thicknesses of disc substrates are set to two kinds ofthicknesses.

FIG. 1 is a constructional diagram of an optical disc apparatus in thefirst embodiment of the invention. FIG. 2 is a plan view showing anarrangement of a main section of the embodiment FIGS. 3A and 3B areperspective views of cartridges of optical discs in the embodiment.

In FIGS. 1 to 3A and 3B, reference numeral 1 denotes a first or secondoptical disc. Thicknesses of disc substrates of both of the first andsecond optical discs are different. Reference numeral 2 denotes acartridge which encloses the optical disc 1 and protects. The cartridge1 is made of plastics or the like. Reference numeral 3 denotes a firstoptical head and 5 indicates a second optical head. Each of the opticalheads is constructed by a converging optical system comprising: anobjective lens; a semiconductor laser; a photo detector; a beamsplitter; and the like (all of the above components are not shown). Eachof the optical head detects an information signal, a focusing errorsignal, and a tracking error signal which have been recorded on theoptical disc 1 on the basis of an intensity or an intensity distributionof the reflected lights of a laser beam irradiated onto the optical disc1 and generates a photo detection signal to the outside. An informationsignal is recorded onto or erased from the optical disc 1 by modulatingan intensity of the laser beam. Both of the optical heads have bases tohold the above optical devices and actuators. A reproduction informationsignal, a focusing error signal, and a tracking error signal which aregenerated from the photo detector of the first optical head 3 areexpressed by S₁, F₁, and T₁, respectively. Similar signals which aregenerated from the photo detector of the second optical disc 5 are alsoexpressed by S₂, F₂, and T_(S), respectively. Reference numeral 4denotes a first linear motor which is arranged below the optical disc 1and moves the first optical head 3 in the radial direction of the discin parallel with the disc surface. Reference numeral 6 denotes a secondlinear motor which is arranged below the optical disc 1 so as to facethe first linear motor 4 and moves the second optical head 5 in a mannersimilar to the first optical head 3.

As shown in FIG. 2, the second linear motor 6 is extended until thefurther outside of the outermost peripheral portion of the optical disc1. Therefore, when the second optical head 5 moves to the outermostside, the optical head 5 is projected from the lower surface of theoptical disc. Reference numeral 7 denotes a discrimination hole formedon the surface of the cartridge 2.

The cartridge in the embodiment will now be described with reference toFIGS. 3A and 3B. The discrimination hole 7 is closed in the case wherethe optical disc 1 enclosed in the cartridge is the first optical discshown in FIG. 3A and is open in the case where it is the second opticaldisc shown in FIG. 3B. Reference numeral 23 denotes a slide shutter.Since the optical disc apparatus of the embodiment has two opticalheads, two slide shutters are provided. When the cartridges are removedfrom the optical disc apparatus, the slide shutters are closed toprotect the internal discs from dusts.

Reference numeral 8 denotes a light emitting diode (hereinafter,abbreviated to an LED) which is arranged so as to be located over thediscrimination hole 7 when the cartridge 2 has been loaded into theoptical disc apparatus of the embodiment. Reference numeral 9 denotes aphoto diode arranged at a position so as to face the LED 8 through thecartridge 2. The photo diode 9 generates a detection signal to a systemcontroller 22, which will be explained hereinlater. Reference numeral 10denotes a first selector for selecting either one of the first group ofphoto detection signals (S₁, F₁, and T₁) of the first optical head 3 andthe second group of photo detection signals (S₂, F₂, and T₂) of thesecond optical head 5 and for outputting the selected signals. Referencenumeral 11 denotes a tracking control circuit which receives thetracking error signal T₁ or T₂ from among the output signals of thefirst selector 10 and generates a tracking actuator driving signal to asecond selector 12, which will be explained hereinlater. Further,reference numeral 12 denotes the second selector to select either anactuator of the first optical head 3 or an actuator of the secondoptical head 5 as a destination to which the tracking actuator drivingsignal is supplied. Reference numeral 13 denotes a focusing controlcircuit which receives the focusing error signal F₁ or F₂ from among thesignals which are generated from the first selector 10 and generates afocusing actuator driving signal to a third selector 14, which will beexplained hereinlater. Further, reference numeral 14 denotes the thirdselector for selecting either the actuator of the first optical head 3or the actuator of the second optical head 5 as a destination to whichthe focusing actuator driving signal is supplied. Reference numeral 15denotes a linear motor control circuit which supplies a driving signalto a fourth selector 16, which will be explained hereinlater, by acontrol signal which is supplied from the system controller 22, whichwill be described hereinlater. Reference numeral 16 denotes the fourthselector to select either the first linear motor 4 or the second linearmotor 6 as an output destination of the driving signal supplied from thelinear motor control circuit 15. Reference numeral 17 denotes a spindlecontrol circuit which receives the reproduction information signal S₁ orS₂ and generates a control current to a spindle motor 18, which will beexplained hereinlater. Reference numeral 18 denotes the spindle motor torotate the optical disc 1. Reference numeral 19 denotes a signalprocessing circuits for executing signal processes such as demodulationand/or decoding or the like to the input signal S_(l) or S₂, forconverting into an audio signal or the like, or for outputting theinformation signal to be recorded onto the optical disc 1 to an LDdriving circuit 20, which will be explained hereinlater. Referencenumeral 20 denotes the LD driving circuit to supply a driving currentfor allowing laser beams to be emitted from the semiconductor lasers ofboth of the optical heads to a fifth selector 21, which will beexplained hereinlater. Reference numeral 21 denotes the fifth selectorto switch the output destination of the above driving current to eitherone of the first and second optical heads 3 and 5 in accordance with acontrol signal from the system controller 22, which will be explainedhereinlater. The system controller 22 generates the driving current forallowing the LED 8 to emit the light and receives the detection signalfrom the photo diode 9 and generates control signals to the first tofifth selectors 10, 12, 14, 16, and 21. The system controller 22 alsogenerates control signals to the linear motor control circuit 15 and thesignal processing circuit 19. In FIG. 1, switching terminals in thefirst to fifth selectors 10, 12, 14, 16 and 21 on the first head sideare designated by “A” and those on the second head side are designatedby “B” in the diagram.

The first optical disc is a CD or an optical disc having a recordingdensity similar to that of the CD and a thickness of the disc substrateis set to d₁ as shown in FIG. 4A. For instance, assuming that theoptical disc shown in FIG. 4A is a CD, d₁=1.2 mm. The second opticaldisc is an optical disc which can execute a recording at a higherdensity than that of the first optical disc and a thickness of the discsubstrate assumes d₂ as shown in FIG. 4B. To reduce the aberration ofthe converged spot due to a tilt error, the thickness d₂ of the secondoptical disc is designed so as to be smaller than d₁. For example,d₂=0.3 mm.

The first optical head 3 has, for instance, a semiconductor laser of awavelength of 780 nm and an objective lens of an NA of 0.45, so that thelaser beam can be converged to a spot diameter φ of about 2.1 μm. Inaddition, an optical design of the objective lens has been made so as tocorrect the aberration by the disc substrate of the thickness d₁. Thesecond optical head 5 has, for instance, the same semiconductor laser asthat of the first optical head and an objective lens of an NA of 0.8, sothat the laser beam can be converged until a spot diameter of 1.2 μm.Moreover, an optical design of the objective lens has been made so as tocorrect the aberration by the disc substrate of the thickness d₂.

The operation of the optical disc apparatus of the embodimentconstructed as mentioned above will now be described hereinbelow.

First, the case where the cartridge 2 enclosing the second optical dischas been loaded into the optical disc apparatus of the embodiment willbe described. When the cartridge 2 is loaded, the LED 8 emits the lightand the photo diode 9 detects the presence or absence of a transmissionlight which passes through the discrimination hole 7. As alreadydescribed in conjunction with FIGS. 3A and 3B, since the discriminationhole 7 is open in the case of the second optical disc, the photo diode 9detects the transmission light and generates a detection signal to thecontroller 22. The controller 22 determines that the disc in the loadedcartridge 2 is the second optical disc, so that the controller 22supplies control signals to the first to fifth selectors 10, 12, 14, 16,and 21 so as to select the terminals B on the second optical head side.Thus, the semiconductor laser of the second optical head 5 is selectedas an output destination of the driving current which is supplied fromthe LD driving circuit 20. The photo detector of the second optical head5 is selected as an input destination of the tracking control circuit11, focusing control circuit 13, spindle control circuit 17, and signalprocessing circuit 19. The actuator of the second optical head 5 isselected as an output destination of the actuator driving signals of thetracking control circuit 11 and focusing control circuit 13. The secondlinear motor 6 is selected as an output destination of the drivingcurrent of the linear motor control circuit 15.

Therefore, the second optical head 5 irradiates the laser beam andconverges onto an information track on the optical disc 1 without anaberration. Simultaneously, the second optical head 5 detects thereflected lights from the disc and generates the information signal S₂,focusing error signal F₂, and tracking error signal T₂. Those signalsare supplied through the first selector 10 to the respective circuits.That is, the signal S₂ is supplied to the spindle control circuit 17 andsignal processing circuit 19. The signal F₂ is supplied to the focusingcontrol circuit 13. The signal T₂ is supplied to the tracking controlcircuit 11. The tracking control circuit 11 produces the trackingactuator driving signal in accordance with the signal T₂ and supplies tothe actuator of the second optical head 5 through the second selector12, thereby eliminating the tracking error. In a manner similar to theabove, the focusing control circuit 13 also produces the focusingactuator driving signal in accordance with the signal F₂ and supplies tothe actuator of the second optical head 5 through the third selector 14,thereby eliminating the focusing error. The linear motor control circuit15 generates the driving current to the linear motor 6 in response tothe control signal from the system controller 22, thereby moving thesecond optical head 5 in the inner or outer rim direction of the opticaldisc 1. The spindle control circuit 17 extracts a clock component fromthe information signal S₂ and controls the spindle motor 18, therebyrotating the optical disc 1 at a constant linear velocity (CLV) or aconstant angular velocity (CAV) or the like. The signal processingcircuit 19 executes signal processes such as demodulation, decoding, andthe like to the information signal S₂ in the reproducing mode andgenerates to the outside as audio or video signals or the like. On theother hand, the signal processing circuit 19 executes signal processessuch as encoding, modulation, and the like to the audio or video signalsor the like which have been supplied from the outside in the recordingmode and generates to the LD driving circuit 20 as a recording signal.Until the cartridge 2 is loaded, the second optical head 5 records orreproduces the information signal onto/from the second optical disc 1.

On the other hand, in the case where the cartridge 2 enclosing the firstoptical disc has been loaded into the optical disc apparatus of theembodiment, since the discrimination hole 7 is closed, the photo diode 9doesn't detect the transmission light. Therefore, the system controller22 determines that the disc in the cartridge 2 is the foregoing firstoptical disc. Thus, the controller 22 generates control signals to thefirst to fifth selectors 10, 12, 14, 16, and 21 so as to select theterminals A on the first optical head side. Therefore, the semiconductorlaser of the first optical head 3 is selected as an output destinationof the driving current which is supplied from the LD driving circuit 20.The photo detector of the first optical head 3 is selected as an inputdestination of the tracking control circuit 11, focusing control circuit13, spindle control circuit 17, and signal processing circuit 19. Theactuator of the first optical head 3 is selected as an outputdestination of the actuator driving signals of the tracking controlcircuit 11 and focusing control circuit 13. The first linear motor 4 isselected as an output destination of the driving current of the linearmotor control circuit 17. Therefore, the first optical head 3 irradiatesthe laser beam and converges onto the information track on the opticaldisc 1 without an aberration. Simultaneously, the reflected lights fromthe disc are detected and generated as the information signal S₁,focusing error signal F₁, and tracking error signal T₁. The abovesignals are supplied through the first selectors 10 to the respectivecircuits That is, the signal S₁ is supplied to the spindle controlcircuit 17 and signal processing circuit 19. The signal F₁ is suppliedto the focusing control circuit 13. The signal T₁ is supplied to thetracking error detecting circuit 11. The subsequent operations aresimilar to those in the case of the second optical disc mentioned above.

In the case where the objective lens of the second optical head is alens of a high NA and a short operating distance, it is necessary to setan interval between the second optical head 5 and the surface of theoptical disc 1 to be fairly narrower than that in the case of the firstoptical head 5. Therefore, while the first optical disc is loaded, thecontroller 22 controls the second linear motor 6, thereby moving thesecond optical head 5 to the outside of the disc surface as shown inFIG. 2. Due to this, it is possible to prevent that the objective lensof the second optical head 5 collides with the optical disc 1 due to asurface oscillation.

As described above, according to the embodiment, by providing the firstoptical head 3 and second optical head 5 having convergence opticalsystems corresponding to the thicknesses of the disc substrate of thefirst and second optical discs, the signal can be preferably recorded,reproduced, or erased by the optical head suitable for the thickness ofeach disc substrate. Moreover, by providing the discrimination hole 7formed on the cartridge 2 and the disc discriminating means comprisingthe LED 8 to detect the opening/closure of the discrimination hole 7 andthe photo diode 9, merely by loading the cartridge 2, each of theoptical heads can be automatically accurately selected.

FIG. 5 shows a constructional diagram of an optical disc apparatusaccording to the second embodiment of the invention. In the diagram, thesecond embodiment differs from the foregoing first embodiment withrespect to a point that a third optical head 30 is arranged in place ofthe first and second optical heads 3 and 5 and a point that the second,third, and fourth selectors 12, 14, and 16 are omitted. That is, thetracking actuator driving current which is generated from the trackingcontrol circuit 11 and the focusing actuator driving current which isgenerated from the focusing control circuit 13 are directly supplied tothe third optical head 30. The driving current which is generated fromthe linear motor control circuit 15 is directly supplied to the firstlinear motor 4.

Further, FIG. 6 shows a detailed constructional diagram of the thirdoptical head 30 in the second embodiment of the invention. In thediagram, reference numeral 1 denotes the first or second optical disc;32 a first semiconductor laser as a light source; 33 a first collimatinglens for converting a laser beam from the first semiconductor laser 32into a parallel beam; 34 first beam splitter to divide the beam into twobeams; 35 a first mirror to change the direction of the beam; 36 a firstobjective lens to converge the beam onto the optical disc 1; 37 a firstdetecting lens to converge the reflected lights which have been dividedby the beam splitter 34; and 38 a first photo detector to obtain aninformation reproduction signal, a focusing error signal, and a trackingerror signal from the converged reflected lights. The above components32 to 38 construct a first converging optical system 31.

Reference numeral 39 denotes a lens holder to hold the first objectivelens 36 and a second objective lens 46, which will be explainedhereinlater. Reference numeral 40 denotes an actuator to support thelens holder 39. The driving currents are supplied from the trackingcontrol circuit 11 and focusing control circuit 13 to the actuator 40.

Reference numeral 42 denotes a second semiconductor laser as a lightsource; 43 a second collimating lens to convert a laser beam from thesecond semiconductor laser 42 into a parallel beam; 44 a second beamsplitter to divide the beam into two beams; 45 a second mirror to changethe direction of the beam; 46 the second objective lens to converge thebeam onto the optical disc 1; 47 a second detecting lens to converge thereflected lights which have been divided by the second beam splitter 44;and 48 a second photo detector to obtain the information reproductionsignal, focusing error signal, and tracking error signal from theconverged reflected lights. The above components 42 to 48 construct asecond converging optical system 41. The above first converging opticalsystem 31, the second converging optical system 41, and the actuator 40are mounted on a same base member (not shown) and construct the thirdoptical head 30. The base member is ordinarily made of aluminum or thelike and is attached to the first linear motor 4.

In a manner similar to the case which has been described in the firstembodiment of the invention, for instance, the first objective lens 36is designed so that NA=0.45 and the aberration by the disc substrate ofa thickness d₁ is corrected. The second objective lens 46 is designed sothat, for instance, NA=0.8 and the aberration by the disc substrate of athickness d₂ is corrected.

The operation of the optical disc apparatus of the embodimentconstructed as mentioned above will now be described hereinbelow. First,the case of the second optical disc will be explained. When thecartridge 2 is loaded into the optical disc apparatus of the embodiment,the LED 8 emits the light and the photo diode 9 detects the presence orabsence of the transmission light which passes through thediscrimination hole 7. If the transmission light has been detected, thesystem controller 22 determines that the disc in the loaded cartridge 2is the second optical disc, so that the controller 22 selects the secondconverging optical system 41 of the third optical head 30. That is, thesystem controller 22 generates control signals to the first and fifthselectors 10 and 21 so as to make the second semiconductor laser 42 andthe second photo detector 48 operative. The laser beam emitted from thesecond semiconductor laser 42 is converted into the parallel beam by thesecond collimating lens 43. The parallel beam is converged onto thesecond optical disc 1 by the second objective lens 46 through the secondbeam splitter 44 and the second mirror 45. The laser beam reflected bythe disc is again converted into the parallel beam by the secondobjective lens 46 and is separated from the optical axis by the secondbeam splitter 44 through the second mirror 45 and is converged onto thesecond photo detector 48 by the second detecting lens 47.

The second photo detector 48 produces the information signal, focusingerror signal, and tracking error signal from the converged reflectedlight from the disc and supplies to the first selector 10. The actuator40 finely moves the lens holder 39 in the tracking and focusingdirections by the driving currents from the tracking control circuit 11and the focusing control circuit 13, thereby accurately converging thelaser beam onto an information track on the optical disc 1.

Since the operations of the first linear motor 4, first selector 10,tracking control circuit 11, focusing control circuit 13, linear motorcontrol circuit 15, spindle control circuit 17, spindle motor 18, signalprocessing circuit 19, LD driving circuit 20, fifth selector 21, andsystem controller 22 are substantially the same as the optical discapparatus of the first embodiment mentioned above, their descriptionsare omitted here.

On the other hand, in the case of the first optical disc, since thephoto diode 9 doesn't detect the transmission light, the systemcontroller 22 decides that the disc in the loaded cartridge 2 is thefirst optical disc mentioned above, so that the controller 22 selectsthe first converging optical system 31 of the third optical head 30.That is, the system controller 22 generates control signals to the firstand fifth selectors 10 and 21 so as to make the first semiconductorlaser 32 and the first photo detector 38 operative. The operation of thefirst converging optical system 31 is substantially the same as that ofthe second converging optical system 41 mentioned above. Until thecartridge 2 is unloaded, the recording reproduction, or erasure of theinformation signal onto/from the first optical disc is executed by thefirst converging optical system 31.

As described above, according to the second embodiment, in addition tothe effect by the first embodiment, since the first converging opticalsystem 31 and the second converging optical system 41 are mounted on thesame base and construct the third optical head 30, the linear motor canbe commonly used as a single part and the number of parts can bereduced.

FIG. 7 shows a constructional diagram of an optical disc apparatusaccording to the third embodiment of the invention. In the diagram, thethird embodiment differs from the second embodiment with respect to apoint that a fourth optical head 50 is arranged in place of the thirdoptical head 30 and a point that the first selector 10 and the fifthselector 21 are omitted. That is, the photo detection signal which isgenerated from the fourth optical head 50 is directly supplied to thetracking control circuit 11, focusing control circuit 13, spindlecontrol circuit 17, and signal processing circuit 19. The drivingcurrent which is generated from the LD driving circuit 20 is directlysupplied to the fourth optical head 50.

Further, FIG. 8 shows a detailed constructional diagram of the fourthoptical head 50 in the third embodiment of the invention. In thediagram, reference numeral 1 denotes the first or second optical disc;32 the first semiconductor laser as a light source; 33 the firstcollimating lens to convert the laser beam from the first semiconductorlaser 32 into the parallel beam; 34 the first beam splitter to dividethe laser beam into two beams; and 35 the first mirror to change thedirection of the beam. The first objective lens 36 and the secondobjective lens 46 are the same as those mentioned in the secondembodiment of the invention. The first objective lens 36 is arrangedbetween the first mirror 35 and the optical disc 1. The second objectivelens 46 is arranged between the first beam splitter 34 and the opticaldisc 1. Moreover, as mentioned in the foregoing embodiment, theaberration of the first objective lens 36 has been corrected incorrespondence to the first optical disc having the disc substrate ofthe thickness d₁. The aberration of the second objective lens 46 hasbeen corrected in correspondence to the second optical disc having thedisc substrate of the thickness d₂. Reference numeral 51 denotes a firstshutter arranged between the first objective lens 36 and the firstmirror 35 and on the optical path which passes through the firstobjective lens 36. Reference numeral 52 denotes a second shutter whichis arranged between the second objective lens 46 and the first beamsplitter 34 and on the optical path which passes through the secondobjective lens 46. The first and second shutters 51 and 52 open or closethe optical paths by control signals from the system controllers 22,respectively. Reference numeral 37 denotes the first detecting lens toconverge the reflected lights which have been divided by the beamsplitter 34. Reference numeral 38 denotes the first photo detector toobtain the information reproduction signal, focusing error signal, andtracking error signal from the converged reflected light.

Among the foregoing component elements, the first objective lens 36constructs the first converging optical system mentioned in the secondembodiment of the invention together with the first semiconductor laser32, the first collimating lens 33, the first beam splitter 34, and thefirst mirror 35. The second objective lens 46 constructs the secondconverging optical system together with the first semiconductor laser32, the first collimating lens 33, the first beam splitter 34, and thefirst mirror 35 which are commonly used for the first converging opticalsystem. The first converging optical system is mounted onto a commonbase (not shown) together with the first and second shutters 5 and 52,thereby constructing the fourth optical head 50. Since the lens holder39 and the actuator 40 have the same construction as those in the thirdoptical head 30 in the second embodiment, their descriptions are omittedhere. The fourth optical head 50 is attached to the first linear motor4.

The operation of the optical disc apparatus in the embodiment with theabove construction will now be described hereinbelow. The kind ofoptical disc is detected in a manner similar to the above. When thesystem controller 22 determines that the disc in the loaded cartridge 2is the second optical disc, the controller 22 generates control signalsto the first and second shutters 51 and 52 of the fourth optical head50. When the control signals are supplied, the first shutter 51 isclosed and the second shutter 52 is open. In the above state, the laserbeam emitted from the first semiconductor laser 32 is converted into theparallel beam by the first collimating lens 33 and is divided into thetransmission light and the reflected light by the first beam splitter34. The transmission light is shut out by the first shutter 51 via thefirst mirror 35. Consequently, only the reflected light passes throughthe second shutter 52 and is converged onto the optical disc 1 by thesecond objective lens 46. The light reflected by the optical disc 1 isagain converted into the parallel light by the second objective lens 46and passes through the second shutter 52 and is reflected and separatedby the first beam splitter 34 and is converged onto the first photodetector 38 by the first detecting lens 37. The first photo detector 38generates the focusing error signal and tracking error signal from theconverged reflected light of the disc and reproduces the informationsignal on the disc. The above operations are executed until thecartridge 2 is unloaded.

Since the operation of the first linear motor 4, tracking controlcircuit 11, focusing control circuit 13, linear motor control circuit15, spindle control circuit 17, spindle motor 18, signal processingcircuit 19, LD driving circuit 20, and system controller 22 are the sameas those in the optical disc apparatus of the foregoing firstembodiment, their descriptions are omitted here.

On the other hand, if the system controller 22 determines that the discin the loaded cartridge 2 is the first optical disc, the first shutter51 is opened and the second shutter 52 is closed. In the above state, inthe transmission light and the reflected light by the first beamsplitter 34, the reflected light is shut out by the second shutter 52and only the transmission light passes through the first shutter 51 andis converged onto the optical disc 1 by the first objective lens 36. Theother operations are executed in a manner similar to those in the caseof the second optical disc.

As mentioned above, according to the third embodiment, in addition tothe effects by the second embodiment, since the first and secondshutters 51 and 52 are provided as light flux selecting means, thesemiconductor laser, collimating lens, beam splitter, detecting lens,and photo detector in each of the converging optical system can becommonly used and the size and weight of the optical head can bereduced. Therefore, assuming that a driving force of the linear motor isthe same, the improvement of the performance such as reduction of theseeking time and the like can be realized.

FIGS. 9A and 9B show a detailed constructional diagram of an opticalhead of an optical disc apparatus in the fourth embodiment of theinvention. In FIG. 9A, since the optical disc 1, first semiconductorlaser 32, first collimating lens 33, first beam splitter 34, secondobjective lens 46, first detecting lens 37, and first photo detector 38are constructed in a manner similar to those in the foregoing fourthoptical head 50, their descriptions are omitted here. Reference numeral56 denotes a lens holder to hold the second objective lens 46; 57 anactuator to which the lens holder 56 is attached; 54 a wave frontcorrecting lens attached to a slider 55, which will be explainedhereinlater, so that the optical axis is in parallel with the opticalaxis of the second objective lens 46; and 55 the slider which supportsthe wave front correcting lens 54 and is arranged so as to transverse inthe plane which is perpendicular to the light flux between the firstbeam splitter 34 and the second objective lens 46, thereby enabling thewave front correcting lens 54 to be moved in such a plane. Moreover,such a movable range is set to a position (shown by P₁ in the diagram)where the wave front correcting lens 54 is perfectly deviated out of thelight flux or a position (shown by P₂ in the diagram) where the opticalaxis of the slider 55 coincides with the optical axis of the secondobjective lens 46. The above-mentioned component elements are attachedto a base (not shown) and construct a fifth optical head 53.

FIG. 9B is a plan view when the wave front correcting lens 54 and theslider 55 are seen from the direction of the optical axis. In thediagram, the lens 54 is movable in the directions shown by arrows. Thewave front correcting lens 54 has been designed in a manner such that asynthetic optical system with the second objective lens 46 is identicalto the foregoing first objective lens. That is, the lens 54 has beendesigned so as to correct the aberration by the disc substrate of thefirst optical disc. In the fifth optical head 53, the second objectivelens 46 constructs the second converging optical system mentioned in thesecond embodiment of the invention together with the first semiconductorlaser 32, first collimating lens 33, and first beam splitter 34 and canbe also regarded such that they construct the first converging opticalsystem by adding the wave front correcting lens 54 to the secondconverging optical system.

Since a whole construction of the optical disc apparatus in the fourthembodiment is substantially the same as that of the optical discapparatus of the third embodiment shown in FIG. 7 mentioned above, itsdescription is omitted here.

The operation of the optical disc apparatus in the fourth embodimentwith the above construction will now be described hereinbelow withrespect to only the fifth optical head 53. The kind of optical disc isdetected in a manner similar to the above. If the system controller 22determines that the disc in the loaded cartridge 2 is the second opticaldisc, the controller 22 generates a control signal to the slider 55.When the control signal is supplied, the slider 55 moves the wave frontcorrecting lens 54 to the position P₁. The laser beam emitted from thefirst semiconductor laser 32 is converted into the parallel light by thefirst collimating lens 33 and is reflected by the first beam splitter 34and is converged onto the optical disc 1 by the second objective lens46. The light reflected by the optical disc 1 is again converted intothe parallel light by the second objective lens 46. The parallel lightpasses through the first beam splitter 34 and is converged onto thefirst photo detector 38 by the first detecting lens 37. The first photodetector 38 generates a photo detection signal in a manner similar tothe above. The above operations are executed until the optical disc 1 isunloaded.

On the other hand, if the system controller 22 decides that the disc inthe loaded cartridge 2 is the first optical disc, the slider 55 movesthe wave front correcting lens 54 to the position P₂. Thus, the laserbeam emitted from the first semiconductor laser 32 passes through thewave front correcting lens 54 and the second objective lens 46 and isconverged onto an information track on the optical disc 1 without anaberration. Thus, the operations similar to those in the case of thesecond optical disc are executed.

As mentioned above, according to the embodiment, in addition to theeffects by the second embodiment, since the wave front correcting lens54 serving as an aberration correcting means is held by the slider 55and is movably arranged, the objective lens can be commonly used and atotal mass which must be moved by the actuator 57 can be reduced. Thus,a burden to the driving force of the actuator can be reduced and a lowelectric power consumption can be accomplished.

The optical head 50 in the fourth embodiment has the second objectivelens 46 corresponding to the optical disc having the disc substrate ofthe thickness d₂ and, further, corrects the aberration to the opticaldisc having the disc substrate of the thickness d₁ by the wave frontcorrecting lens 54. However, an opposite construction can be also used.Namely, the above effect is also derived by a construction such that thefirst objective lens 36 corresponding to the optical disc of the discsubstrate of the thickness d₁ is used in place of the second objectivelens 46 and a wave front correcting lens which has been designed so asto correct the aberration due to the disc substrate of the thickness d₂is provided.

Although the above three embodiments have been described with respect tothe case where there are two kinds of thicknesses of the discsubstrates, the invention, can be also applied to the case of three ormore kinds of thicknesses of the disc substrates. In such a case, thenumber of optical elements such as objective lenses and the like isincreased in accordance with the number of kinds of thicknesses of thedisc substrates. With respect to the discriminating means of the opticaldisc, three or more kinds of optical discs can be discriminated by, forinstance, checking a plurality of discrimination holes which are formedin the cartridge. For example, by forming n discriminating holes, 2^(n)kinds of optical discs can be discriminated.

Further, although the discrimination hole 7 formed on the cartridge 2,the LED 8, and the photo diode 9 have been used as disc discriminatingmeans, paints of different reflectances can be also coated onto thesurface of the cartridge 2 in place of the discrimination hole or amechanical switch or the like can be also used in place of the LED andthe photo diode.

Further, a difference between thicknesses of the disc substrates can bealso directly discriminated by a reflected laser beam from the discswithout using the cartridge. For example, in the case of the convergingoptical system corresponding to the thin disc substrate, a trackingerror signal cannot be ordinarily obtained from an optical disc of athick disc substrate due to a spherical aberration of the convergingbeam. Consequently, two optical discs having different thicknesses canbe discriminated by checking the presence or absence of the trackingerror signal. In such a case, there is an excellent effect such that theapparatus is simplified because there is no need to use the detectingmeans such as LED and photo diode and the like.

The optical head in each of the above-described optical disc apparatuseshas been constructed by a conventional optical system using theobjective lens made of a quartz glass or the like. An optical head ineach of optical disc apparatuses of embodiments, which will be explainedhereinlater, differs from the above optical head and is constructed byforming an optical system onto a thin film waveguide.

FIG. 10 is a block diagram showing a construction of an optical discapparatus according to the fifth embodiment of the invention. Further,FIG. 11 is a schematic perspective view showing a construction of anoptical head of the optical disc apparatus in the fifth embodiment ofthe invention. Since a construction shown in FIG. 10 is substantiallythe same as that of the optical disc apparatus in the second embodimentof the invention shown in FIG. 5 except that a sixth optical head 60 isused, its description is omitted here. The sixth optical head 60 shownin FIG. 11 will now be described in detail hereinbelow.

In FIG. 11, reference numeral 1 denotes the same optical disc as thatdescribed in the foregoing embodiments. Reference numeral 200 denotes aninformation track formed on the optical disc 1. Reference numeral 61denotes a substrate formed by LiNbO₃ or the like. The substrate 61 isattached to a head base through a focusing actuator and a trackingactuator and constructs the sixth optical head 60 together with them.Since the focusing actuator, tracking actuator, and head base which haveconventionally been well known can be used as those components, theirdetailed description and the drawings are omitted here. Referencenumeral 62 denotes an optical waveguide formed on the substrate 51 by Tidiffusion or the like; 63 a first semiconductor laser coupled to an edgesurface of the optical waveguide 62; and 64 a first waveguide lensarranged on an optical path of the waveguide light which has beenemitted from the first semiconductor laser 63 and entered the opticalwaveguide 61. For instance, a Fresnel lens formed by an electron beamlithography can be used as a lens 64. Reference numeral 65 denotes afirst converging grating coupler formed on the optical path of theparallel waveguide light. The coupler 65 emits the waveguide light to aposition out of the optical waveguide 62 and converges onto the opticaldisc 1. The first converging grating coupler 65 is a grating having achirp (irregular period) by a curve formed on the waveguide by electronbeam direct drawing or the like. Reference numeral 66 denotes a firstbeam splitter which is arranged between the first waveguide lens 64 andthe first converging grating coupler 65 and separates the waveguidelight which has been returned into the optical waveguide 62 through thefirst converging grating coupler 65 after it had been reflected by theoptical disc 1. Reference numeral 67 denotes a first waveguideconverging lens which is arranged on the optical path of the returnwaveguide light which has been separated by the first beam splitter 66and converges the return light. Reference numeral 68 denotes a firstphoto detector which is coupled to the side surface of the opticalwaveguide 62 and detects the return waveguide light which has beenconverged by the first waveguide converging lens 67.

Similarly, reference numeral 69 denotes a second semiconductor lasercoupled to the edge surface of the optical waveguide 62; 70 a secondwaveguide lens arranged on the optical path of the waveguide light whichhas been emitted from the second semiconductor laser 69 and entered theoptical waveguide 61; and 71 a second converging grating coupler formedon the optical path of the parallel waveguide light. The coupler 71emits the waveguide light to a position out of the optical waveguide 62and converges onto the optical disc 1. Reference numeral 72 denotes asecond beam splitter which is arranged between the second waveguide lens70 and the second converging grating coupler 71 and separates thewaveguide light which has been returned into the optical waveguide 62through the second converging grating coupler 71 after it had beenreflected by the optical disc 1. Reference numeral 73 denotes a secondwaveguide converging lens which is arranged on the optical path of thereturn waveguide light which has been separated by the second beamsplitter 72 and converges the return waveguide light. Reference numeral74 denotes a second photo detector which is coupled to the side surfaceof the optical waveguide 62 and detects the return waveguide lightconverged by the second waveguide converging lens 73.

A curve chirp grating of the first converging grating coupler 65 hasbeen designed in a manner such that, for instance, NA=0.45 and theemission light can be converged until a diffraction limit and theaberration due to the disc substrate of the thickness d₁ can becorrected. The second converging grating coupler 71 has been designed ina manner such that, for example, NA=0.8 and the aberration due to thedisc substrate of the thickness d₂ can be corrected.

The first and second beam splitters 66 and 72 are attached at positionswhich are deviated so that the reflected light of each beam splitterdoes not enter the other beam splitter as a stray light.

Such an optical waveguide and a waveguide type device have beendescribed in detail in, for example, Nishihara, Haruna, and Saihara,“Optical Integrated Circuit”, Ohm Co., Ltd., 1985, or the like. In theinvention, both of the above well-known optical waveguide and waveguidetype device can be used in the optical waveguide 62 or the like.

The operation of the optical head in the fifth embodiment with the aboveconstruction will now be described hereinbelow.

If the optical disc 1 is the first optical disc, the driving current issupplied to the first semiconductor laser 63. Then, the laser 63 emits alaser beam from one edge surface of the optical waveguide 62. The laserbeam propagates as a waveguide light. The waveguide light is convertedinto the parallel light by the first waveguide lens 64. The parallellight transmits the first beam splitter 66 and subsequently enters thefirst converging grating coupler 65. The coupler 65 extracts theparallel light out of the optical waveguide 62 and converges onto theinformation track 200 on the first optical disc 1. The reflected lightfrom the disc surface again enters the optical waveguide 62 through thefirst converging grating coupler 65 and propagates as a return waveguidelight in the opposite direction. Further, the return waveguide light isreflected in the direction of the first waveguide converging lens 67 inthe first beam splitter 66. The lens 67 converges the return waveguidelight onto the first photo detector 68. The first photo detector 68detects the information signal and the servo signals such as focusingerror signal, tracking error signal, and the like which have beenrecorded on the first optical disc 1 on the basis of an intensity and anintensity distribution of the return waveguide light and generates tothe outside. By modulating the driving current which is supplied to thefirst semiconductor laser 63, the sixth optical head 60 emits theintensity modulated laser beam, thereby recording or erasing theinformation signal onto/from the first optical disc 1.

On the other hand, if the optical disc 1 is the second optical disc, theoperations similar to those in the case of the foregoing first opticaldisc are executed by the second semiconductor laser 69, second waveguidelens 70, second converging grating coupler 71, second beam splitter 72,second waveguide converging lens 73, and second photo detector 74.

The substrate 61 is supported from the head base by a focusing actuatorand a tracking actuator. The position of the substrate 61 itself iscontrolled by the foregoing servo signals so that the laser beam isaccurately irradiated onto the information track 200 on the disc.

According to the fifth embodiment as mentioned above, by providing thefirst converging grating coupler 65 which is formed on the opticalwaveguide 62 and corresponds to the thickness of the disc substrate ofthe first optical disc and the second converging grating coupler 71which is formed on the optical waveguide 62 and corresponds to thethickness of the disc substrate of the second optical disc, a desiredone of the couplers 65 and 71 can be independently used in accordancewith the kind of disc, so that the aberration of the converged spot canbe corrected in accordance with the thickness of the disc substrate andthe signal can be preferably recorded, reproduced, or erased. Moreover,since the optical waveguide device having the converging gratingcouplers is used, the size and weight of the optical head can bereduced.

Although the thickness of the disc substrate has been set into two kindsof thicknesses in the fifth embodiment, the invention can be alsoapplied to three or more kinds of thicknesses of disc substrates. Insuch a case, the number of component elements on the substrate 61 isincreased in accordance with the number of thicknesses.

In the sixth optical head 60, either one of the semiconductor lasers hasbeen allowed to emit the light. However, it is also possible to allowboth of the semiconductor lasers to simultaneously emit the lights. Insuch a case, by designing two converging grating couplers for theoptical disc having the same substrate thickness, two tracks on theoptical disc 1 can be simultaneously reproduced or recorded. Thus, thereis an excellent effect such that the reproducing or recording transferspeed can be doubled.

FIG. 12 is a schematic perspective view showing a construction of anoptical head of an optical disc apparatus according to the sixthembodiment of the invention.

In the diagram, the sixth embodiment has substantially the sameconstruction as that of the sixth optical head 60 shown in FIG. 11except a third beam splitter 81 and a waveguide mirror 82 and the sameparts and components as those shown in FIG. 11 are designated by thesame reference numerals. That is, an optical head of the sixthembodiment, namely, a seventh optical head 80 is constructed in thefollowing manner. In place of the second semiconductor laser 69 and thesecond waveguide lens 70 in the sixth optical head 60 shown in FIG. 11,the third bean splitter 81 is arranged on the optical path between thewaveguide lens 64 and the beam splitter 66. In the two waveguide lightsdivided by the third beam splitter 81, the waveguide mirror 82 isarranged in the direction of the waveguide divided in the directiondifferent from the direction of the first beam splitter 66 and theposition of the mirror 82 is set to a position where the waveguide lightreflected by the waveguide mirror 82 passes through the second beamsplitter 72.

The operation of the seventh optical head 80 with the above constructionwill now be described hereinbelow.

The driving current is supplied to the first semiconductor laser 63. Thelaser 63 emits a laser beam from one edge surface of the opticalwaveguide 62. The laser beam propagates as a waveguide light. Thewaveguide light is converted into the parallel light by the firstwaveguide lens 64 and is divided into the transmission light and thereflected light by the third beam splitter 81. The transmission light istransferred to the first converging grating coupler 65 through the firstbeam splitter 66. The reflected light is reflected by the waveguidemirror 82 and enters the second converging grating coupler 71 throughthe second beam splitter 72. The subsequent operations are executed in amanner similar to those of the sixth optical heat 60 in the fifthembodiment of the invention.

According to the sixth embodiment as mentioned above, in addition to theeffects by the foregoing fifth embodiment, by dividing the waveguidelight emitted from one semiconductor laser into two lights by the thirdbeam splitter 81 and guiding to the respective converging gratingcouplers, the number of semiconductor lasers which are used can bereduced.

Although the sixth embodiment has been described on the assumption thatthe number of thicknesses of the disc substrates is set to two kinds ofthicknesses, the invention can be also applied to three or more kinds ofthicknesses of the disc substrates. Now, assuming that the number ofkinds of thicknesses of the disc substrates is equal to N, it issufficient to use N converging grating couplers and (N−1) beam splittersfor dividing the waveguide light emitted from the semiconductor laser.To equalize all of light quantities of the laser beams which areconverged onto the discs, it is preferable to design the beam splittersso as to set division ratios of the light quantities of the beamsplitters as follows. 1:N − 1 1:N − 2 1:N − 3 ⋮ 1:1

FIG. 13 is a schematic perspective view showing a construction of anoptical head of an optical disc apparatus according to the seventhembodiment of the invention. FIG. 14 is a block diagram showing aconstruction of the optical disc apparatus.

A construction of the optical head in FIG. 13 will be first described indetail.

In the diagram, since the optical disc 1, information track 200,substrate 61, optical waveguide 62, first semiconductor laser 63 andfirst waveguide lens 64 are fundamentally identical to the componentelements in the seventh optical head 80 shown in FIG. 12, their detaileddescriptions are omitted here. Reference numeral 91 denotes an SAW(surface acoustic wave) transducer arranged on the optical waveguide 62so that a surface acoustic wave generated by the SAW transducer crossesthe optical path of the waveguide light emitted from the first waveguidelens 64. The SAW transducer 91 is constructed by a cross fingerelectrode comprising a piezoelectric transducer of ZnO or the like.Reference numeral 91 indicates a surface acoustic wave generated by theSAW transducer 91; 96 a third converging grating coupler formed on theoptical path of the waveguide light which has been diffracted by such asurface acoustic wave 92 and propagates in the first direction; and 97 afourth converging grating coupler which is likewise formed on theoptical path of the waveguide light propagating in the second direction.Each of the couplers 96 and 97 emits the waveguide light to a region outof the optical waveguide 62 and converges onto the optical disc 1.Reference numeral 93 denotes a fourth beam splitter which is arrangedbetween the first waveguide lens 64 and the progressing path of thesurface acoustic wave 92 and reflects the waveguide light returned intothe optical waveguide 62 through the third or fourth converging gratingcouplers 96 and 97 after it had been reflected by the optical disc 1.Reference numeral 94 denotes a third waveguide converging lens which isarranged on the optical path of the return light reflected by the fourthbeam splitter 93 and converges the return light and 95 indicates a thirdphoto detector which is coupled to the side surface of the opticalwaveguide 62 and detects the return light converged by the thirdwaveguide converging lens 94.

The above SAW transducer has also been described in detail in theforegoing “Optical Integrated Circuit” or the like and both of thewell-known optical waveguide and waveguide type device described in theabove literature can be also obviously used.

The operation of the optical head in the seventh embodiment with theabove construction will now be described hereinbelow.

The first semiconductor laser 63 emits a laser beam from one edgesurface of the optical waveguide 62. The laser beam propagates as awaveguide light. The waveguide light is converted into the parallellight by the first waveguide lens 64 and transmits through the fourthbeam splitter 93. After that, the light transverses the surface acousticwave 92 generated from the SAW transducer 91. At this time, thepropagating direction of the parallel waveguide light is changed by anacoustic optical interaction with the surface acoustic wave 92. Since adeflection angle at this time changes in accordance with a frequency ofthe surface acoustic wave 92, the waveguide light can be propagated inany one of the directions of the third and fourth converging gratingcouplers 96 and 97 in accordance with frequencies of high-frequencyvoltages which are applied to the SAW transducer 91 from the outside (itis now assumed that the frequencies of the high-frequency frequencyvoltages are set to f₁ and f₂, respectively). In the case of the firstoptical disc, therefore, the high-frequency voltage of the frequency f₁is applied to the SAW transducer 91 from the outside, thereby allowingthe parallel waveguide light to enter the third converging gratingcoupler 96. The third converging grating coupler 96 extracts theparallel waveguide light to a region out of the optical waveguide 62 andconverges onto the information track 200 on the first optical disc 1.The reflected light from the disc surface again enters the opticalwaveguide 62 through the third grating coupler 96 and propagates as areturn waveguide light in the opposite direction. The progressingdirection of the waveguide light is changed by the surface acoustic wave92 and, after that, the waveguide light is reflected in the direction ofthe third waveguide converging lens 94 by the fourth beam splitter 93.The third waveguide converging lens 94 converges the return light to thethird photo detector 95. The third photo detector 95 detects theinformation signal and the servo signals such as focusing error signal,tracking error signal, and the like which have been recorded on thefirst optical disc 1 on the basis of an intensity and an intensitydistribution of the return light and generates to the outside. Bymodulating the driving current which is supplied to the firstsemiconductor laser 63, an eighth optical head 90 emits the intensitymodulated laser beam, thereby recording or erasing the informationsignal onto/from the first optical disc 1.

On the other hand, in the case of the second optical disc, thehigh-frequency voltage of the frequency f₂ is applied to the SAWtransducer 91 from the outside, thereby allowing the parallel waveguidelight to enter the fourth converging grating coupler 97. The subsequentoperations are executed in a manner similar to those in the case of thefirst optical disc.

The substrate 61 is supported from the head base by a focusing actuatorand a tracking actuator (not shown). The position of the substrate 61itself is controlled by the servo signals so that the laser beam isaccurately irradiated onto the information track 200 on the disc.

An optical disc apparatus having the eighth optical head 90 mentionedabove will now be described with reference to FIG. 14.

In the diagram, the optical disc 1, cartridge 2, first linear motor 4,discrimination hole 7, LED 8, photo diode 9, tracking control circuit11, focusing control circuit 13, linear motor control circuit 15,spindle control circuit 17, spindle motor 18, signal processing circuit19, LD driving circuit 20 and system controller 22 are the same as thosein the optical disc apparatus in the third embodiment according to theinvention. Reference numeral 90 denotes the eighth optical head which isconstructed by the waveguide substrate, focusing actuator, trackingactuator, head base and the like. Reference numeral 85 denotes aconstant voltage generating circuit which receives a control signal fromthe system controller 22 and generates a predetermined voltage V_(i).Reference numeral 86 denotes a V/f converting circuit which receives thevoltage V_(i) from the constant voltage generating circuit 85 andgenerates a high-frequency signal of a frequency f which is proportionalto V_(i). The V/f converting circuit 86 generates a high-frequencysignal of the frequency f₁ when the input voltage V_(i)=V₁ and generatesa high frequency signal of the frequency f₂ when V_(i)=V₂. Referencenumeral 87 denotes an SAW driving circuit to apply a high-frequencyvoltage of the same frequency as the frequency f of the high-frequencysignal supplied from the V/f converting circuit 86 to the SAW transducer91 of the eighth optical head 90.

The operation of the optical disc apparatus in the seventh embodimentwith the above construction will now be described hereinbelow.

First, if the cartridge 2 enclosing the second optical disc has beenloaded into the optical disc apparatus of the seventh embodiment, thesystem controller 22 determines that the disc in the loaded cartridge 2is the second optical disc by the detection signal of the photo diode 9,so that the controller 22 generates a control signal to the constantvoltage generating circuit 85 so as to generate the voltage V₂. The V/fconverting circuit 86 converts the input voltage V₂ into the frequencyf₂, so that the SAW driving circuit 87 applies the high-frequencyvoltage of the frequency f₂ to the SAW transducer 91 of the eighthoptical head 90. Therefore, in the eighth optical head 90, the laserbeam is irradiated from the second converging grating coupler 97 and isconverged without an aberration onto the information track 200 on thesecond optical disc having the disc substrate of the thickness d₂. Atthe same time, the third photo detector 95 of the eighth optical head 90detects a focusing error signal and a tracking error signal from thereflected light from the optical disc and supplies to the trackingcontrol circuit 11 and the focusing control circuit 13. Further, theinformation signal on the disc is supplied to the signal processingcircuit 19 and the spindle control circuit 17.

On the other hand, in the case of the first optical disc, the systemcontroller 22 generates a control signal to the constant voltagegenerating circuit 85 so as to generate the voltage V₁. The V/fconverting circuit 86 converts the input voltage V₁ into the frequencyf₁, so that the SAW driving circuit 87 applies the high-frequencyvoltage of the frequency f₁ to the SAW transducer 91 of the eighthoptical head 90. Therefore, in the eighth optical head 90, a laser beamis emitted from the first converging grating coupler 96 and is convergedwithout an aberration onto the information track 200 on the firstoptical disc having the disc substrate of the thickness d₁. The otheroperations are executed in a manner similar to those in the case of theforegoing second optical disc

According to the embodiment as mentioned above, in addition to theeffects of the above sixth embodiment, the number of semiconductorlasers which are necessary in the eighth optical head 90 is only one andeach of the converging grating couplers does not simultaneously emit thelaser beam, so that an emission power of the semiconductor laser can beefficiently taken out of the converging grating coupler. That is, theoptical head having a transfer efficiency better than that of theseventh optical head 80 in the foregoing sixth embodiment can beprovided.

Further, by arranging the fourth beam splitter 93 between the waveguidelens 64 and the SAW transducer 91, the return lights from the twoconverging grating couplers can be detected by one photo detector.

Although the number of thicknesses of the disc substrates has been setto two kinds of thicknesses in the embodiment, the invention can be alsoobviously applied to three or more kinds of thicknesses of the discsubstrates. In such a case, the number of converging grating couplers isincreased in accordance with the number of kinds of thicknesses and theoptical paths are switched by the SAW transducer 91 in accordance withthe increased number of such couplers.

An optical disc apparatus in the eighth embodiment of the invention willnow be described.

FIG. 15 is a block diagram showing a construction of the optical discapparatus in the eighth embodiment. In the diagram, reference numeral 1denotes the first or second optical disc; 2 the cartridge; 4 the linearmotor; 7 the discrimination hole; 8 the LED; 9 the photo diode; 13 thefocusing control circuit; 15 the linear motor control circuit; 17 thespindle control circuit; 18 the spindle motor; 19 the signal processingcircuit; 20 the LD driving circuit; 22 the system controller; 85 theconstant voltage generating circuit; 86 the V/f converting circuit; and87 the SAW driving circuit. The above component elements are the same asthose in the optical disc apparatus in the seventh embodiment of FIG. 14and their detailed descriptions are omitted here. Reference numeral 90denotes an optical head which is substantially the same as the eighthoptical head 90 mentioned above except that the optical head in theeighth embodiment does not have a tracking actuator. Therefore, theoptical head in FIG. 15 is also referred to as an eighth optical head 90hereinafter for convenience of explanation. Reference numeral 100denotes a tracking error detecting circuit which receives a trackingerror signal from the third photo detector 95 of the eighth optical head90 and generates a tracking error voltage V_(TE) to an adder 101, whichwill be explained hereinafter. Reference numeral 101 denotes the adder.The voltage V_(TE) which is generated from the tracking error detectingcircuit 100 and the voltage V_(i) which is generated from the constantvoltage generating circuit 85 are supplied to the adder 101, so that theadder generates a voltage V₀ (V₀=V_(TE)+V_(i)) to the V/f convertingcircuit 86. The V/f converting circuit 86, SAW driving circuit 87,tracking error detecting circuit 100, and adder 101 construct a trackingcontrol circuit 102. That is, it is the inventive point of the eighthembodiment that the tracking control is executed by using the SAWtransducer 91 of the eighth optical head 90.

The principle of the tracking control of the eighth embodiment will nowbe described hereinbelow with reference to the drawings. FIG. 16 is anenlarged schematic perspective view of the converging grating coupler,SAW transducer, and portion where a surface acoustic wave has beenformed. The waveguide light which enters the converging grating coupleris oscillated between solid lines and broken lines in accordance with amicrochange of the frequency of the surface acoustic wave. Such anoscillation angle is called a deflection angle (shown by θ). Therefore,the emission light from the converging grating coupler is alsooscillated and the converged spot moves. Since the deflecting angle θchanges in almost proportional to the frequency of the surface acousticwave, by changing the frequency in accordance with the tracking erroramount, the converged spot can be accurately positioned onto theinformation track.

The operation of the optical disc apparatus of the embodimentconstructed as shown in FIG. 15 will now be described hereinbelow.First, if the disc in the loaded cartridge 2 is the second optical disc,the system controller 22 controls the constant voltage generatingcircuit 85 so as to set the output voltage V_(i) into V₂. The outputvoltage V_(TE) of the tracking error detecting circuit 100 has beeninitialized to “0”. The adder 101 adds the voltage V_(i) and V_(TE) andgenerates the voltage V₀ (=V₂) to the V/f converting circuit 86. The V/fconverting circuit 86 changes a frequency f_(s) of an output signal inaccordance with the input voltage V₀. The optical disc apparatus hasbeen designed in a manner such that the signal of a frequency f_(s)(=f₁)is generated when V₀=V₁ and the signal of a frequency f_(s)(=f₂) isgenerated when V₀=V₂ and the frequency f_(s) changes in proportion tothe input voltage V₀. Therefore, the V/f converting circuit 86 suppliesa high-frequency signal of the frequency f₂ to the SAW driving circuit87. The SAW driving circuit 87 applies a high-frequency voltage of thefrequency f₂ to the SAW transducer 91 of the eighth optical head 90. Inthe eighth optical head 90, consequently, the laser beam is emitted fromthe fourth converging grating coupler 97 and is converged without anaberration onto the information track on the second optical disc. At thesame time, in the eighth optical head 90, the reflected light from thedisc is detected by the third photo detector 95. A tracking error signalis supplied to the tracking error detecting circuit 100. A focusingerror signal is supplied to the focusing control circuit 13. Theinformation signal is supplied to the spindle control circuit 17 and thesignal processing circuit 19. The tracking error detecting circuit 100produces the tracking error voltage V_(TE) in accordance with a trackdeviation amount of the converged spot on the information track 200 andsupplied to the adder 101. The adder 101 sends the output voltageV₀=V₂+V_(TE) to the V/f converting circuit 86 as mentioned above. Inaccordance with the output voltage V₀, the output signal frequency f_(s)of the V/f converting circuit 86 is deviated from the frequency f₂ by avalue corresponding to the tracking error (assumes d_(f)). As mentionedabove, when the frequency of the driving voltage to the SAW transducer91 changes, the emitting position of the light from the fourthconverging grating coupler 97 changes and the position of the convergedspot on the optical disc 1 changes for the track. Therefore, by settinga converting equation between V₀ and f_(s) of the V/f converting circuit86 so as to allow the converted spot on the optical disc 1 to approachthe track, the tracking error is eliminated. The other operations areexecuted in a manner similar to those of the optical disc apparatus inthe seventh embodiment.

On the other hand, in the case of the first optical disc, the systemcontroller 22 controls the constant voltage generating circuit 85,thereby setting the output voltage V_(i) into V₁. Thus, the V/fconverting circuit 86 generates a high-frequency signal of the frequencyf₁ to the SAW driving circuit 87 and the SAW driving circuit 87 appliesa high-frequency voltage of the frequency f₁ to the SAW transducer 91 ofthe eighth optical head 90. Consequently, in the eighth optical head 90,the laser beam is emitted from the third converging grating coupler 96and is converged without an aberration onto the information track 200 onthe first optical disc. At the same time, the tracking error detectingcircuit 100 supplies the tracking error voltage V_(TE) to the adder 101from the input signal T₁. The input voltage of the V/f convertingcircuit 86 is set to V₀=V₁+V_(TE) and the tracking error can beeliminated in a manner similar to the case of the second optical disc.

FIG. 17 is a graph showing the principle of the tracking control of theembodiment and shows the relations among the V₀ and f_(s) and thedeflection angle of the waveguide light in the eighth optical head 90.As shown in the graph, by varying the V₀ and f_(s) by only an amountwhich is proportional to the tracking error signal from V₁, accordingly,f₁ as a center in the case of the first optical disc or by only anamount which is proportional to the tracking error signal from V₂,accordingly, f₂ as a center in the case of the second optical disc, theoscillation angle of the waveguide light can be finely varied.Therefore, by varying the emitting positions of the light beams from thetwo converging grating couplers, the converged spot can be allowed totrace on the track.

According to the eight embodiment as mentioned above, in addition to theeffects of the foregoing seventh embodiment, the change-over of thewaveguide lights which enter the converging grating couplers and thetracking control can be executed by the SAW transducer 91. Thus, theoptical head can be simplified and the number of manufacturing steps canbe reduced.

Since the surface acoustic wave 92 is located between the fourth beamsplitter 93 and the two converging grating couplers, the returnwaveguide light from the optical disc 1 is not influenced by thetracking control on the optical path after the surface acoustic wave 92.Therefore, the converging portion on the third photo detector is notmoved by the tracking control, so that a deterioration in photodetection signal can be prevented.

In the embodiment, although the SAW transducer has been used as both ofthe optical path switching means and the optical path deflecting meansfor tracking control, the SAW transducer can be also provided for theoptical head only for the tracking control. For instance, it is alsopossible to form the SAW transducer for the sixth optical head 60 in thefifth embodiment or the seventh optical head 80 in the sixth embodimentand to execute the tracking control.

1. An optical recording/reproducing apparatus for recording, reproducingor erasing an information signal by converging a light flux onto/from arecording layer through a transparent disc substrate, comprising: (a) Noptical heads, N being greater than or equal to 2, each comprising:light emitting means, objective lenses, whose aberrations haverespectively been corrected for said N disc substrates having differentthicknesses, each for converging the light flux which is emitted fromthe light emitting means onto the optical disc, and a plurality of photodetecting means each for detecting the reflected light from the opticaldisc; (b) N optical head moving means which are arranged below theoptical disc and move the N optical heads in the radial direction of theoptical disc; (c) disc discriminating means for discriminating thethickness of the disc substrate of the loaded optical disc and forgenerating a discrimination signal in accordance with the result of thediscrimination; and (d) control means for selecting the optical headhaving the objective lens in which the occurrence of the aberration dueto the disc substrate is smallest in accordance with the discriminationsignal, wherein the selected optical head records, reproduces or erasesthe information signal onto/from the optical disc.
 2. An apparatusaccording to claim 1, further comprising backward moving means formoving the non-selected optical heads to the outside of the optical discfor a period of time when the optical head which has been selected bythe control means is recording, reproducing, or erasing the informationsignal.
 3. An apparatus according to claim 1, wherein said discdiscriminating means comprises: a cartridge for enclosing the opticaldisc; a discrimination hole which is formed on the cartridge and whoseopening/closing state differs in correspondence to the thickness of thedisc substrate of the optical disc; and detecting means for detectingthe opening/closing state of the discrimination hole and for generatinga discrimination signal.
 4. An apparatus according to claim 2, whereinsaid disc discriminating means comprises: a cartridge for enclosing theoptical disc; a discrimination hole which is formed on the cartridge andwhose opening/closing state differs in correspondence to the thicknessof the disc substrate of the optical disc; and detecting means fordetecting the opening/closing state of the discrimination hole and forgenerating a discrimination signal.
 5. An apparatus according to claim1, wherein numerical apertures of at least two or more of said Nobjective lenses differ.
 6. An apparatus according to claim 2, whereinnumerical apertures of at least two or more of said N objective lensesdiffer.
 7. An optical recording/reproducing apparatus for recording,reproducing or erasing an information signal by converging a light fluxonto/from a recording layer through a transparent disc substrate,comprising: (a) an optical head having N, N being greater than or equalto 2, converging optical systems each comprising: light emitting means,objective lenses, whose aberrations have respectively been corrected forsaid N disc substrates having different thicknesses, each for convergingthe light flux which is emitted from the light emitting means onto theoptical disc, and a plurality of photo detecting means each fordetecting the reflected light from the optical disc; (b) optical headmoving means which is arranged below the optical disc and moves theoptical head in the radial direction of the optical disc; (c) discdiscriminating means for discriminating the thickness of the discsubstrate of the loaded optical disc and for generating a discriminationsignal in accordance with the result of the discrimination; and (d)control means for allowing the light emitting means, which belongs tothe converging optical system in which the occurrence of the aberrationdue to the disc substrate is smallest in accordance with thediscrimination signal, to emit light, wherein the selected convergingoptical system records, reproduces or erases the information signalonto/from the optical disc.
 8. An apparatus according to claim 7,wherein said disc discriminating means comprises: a cartridge forenclosing the optical disc; a discrimination hole which is formed on thecartridge and whose opening/closing state differs in correspondence tothe thickness of the disc substrate of the optical disc; and detectingmeans for detecting the opening/closing state of the discrimination holeand for generating a discrimination signal.
 9. An apparatus according toclaim 7, wherein numerical apertures of at least two or more of said Nobjective lenses differ.
 10. An optical recording/reproducing apparatusfor recording, reproducing or erasing an information signal byconverging a light flux onto/from a recording layer through atransparent disc substrate, comprising: (a) an optical head including:light emitting means, light flux dividing means which are arranged inthe light flux from the emitting means and divide the emitted light fluxinto N, N being greater than or equal to 2, light fluxes and deflect indifferent directions, N objective lenses, whose aberrations haverespectively been corrected for said N disc substrates having differentthicknesses, for respectively converging said N light fluxes onto theoptical disc, light flux selecting means for selecting one of the Nlight fluxes divided by the light flux dividing means and for allowingsaid light flux to pass, and photo detecting means for detecting thelight fluxes reflected by the optical disc; (b) optical head movingmeans which is arranged below the optical disc and moves the opticalhead in the radial direction of the optical disc; (c) discdiscriminating means for discriminating the thickness of the discsubstrate of the loaded optical disc and for generating a discriminationsignal in accordance with the result of the discrimination; and (d)control means for generating a control signal to the light fluxselecting means in accordance with the discrimination signal and forselecting the light flux which passes through the objective lens inwhich the occurrence of the aberration due to the disc substrate issmallest, wherein the optical head records, reproduces or erases theinformation signal onto/from the optical disc by the selected lightflux.
 11. An apparatus according to claim 10, wherein said discdiscriminating means comprises: a cartridge for enclosing the opticaldisc; a discrimination hole which is formed on the cartridge and whoseopening/closing state differs in correspondence to the thickness of thedisc substrate of the optical disc; and detecting means for detectingthe opening/closing state of the discrimination hole and for generatinga discrimination signal.
 12. An apparatus according to claim 10, whereinnumerical apertures of at least two or more of said N objective lensesdiffer.
 13. An optical recording/reproducing apparatus for recording,reproducing or erasing an information signal by converging a light fluxonto/from a recording layer through a transparent disc substrate,comprising: (a) an optical head including: an optical waveguide formedon a substrate, N light emitting means each for emitting a waveguidelight into said optical waveguide, N being greater than or equal to 2, Nconverging grating couplers, whose aberrations have respectively beencorrected for said N disc substrates having different thicknesses, eachfor emitting the waveguide light supplied from said N light emittingmeans to the outside of the optical waveguide and for allowing thereflected light from the optical disc to enter, and N photo detectingmeans each for detecting reflected light and for generating aninformation signal; (b) optical head moving means which is arrangedbelow the optical disc and moves the optical head in the radialdirection of the optical disc; (c) selecting means for selecting thelight emitting means to be allowed to emit the light from among the Nemitting means; (d) disc discriminating means for discriminating thethickness of the disc substrate of the loaded optical disc and forgenerating a discrimination signal according to the result of thediscrimination; and (e) control means for generating a control signal inaccordance with the discrimination signal, for providing said controlsignal to said selecting means and for allowing the light emitting meansfor emitting the waveguide light into the converging grating coupler inwhich the occurrence of the aberration due to the disc substrate issmallest, wherein the optical head records, reproduces or erases theinformation signal onto/from the optical disc by the light flux from theselected light emitting means.
 14. An apparatus according to claim 13,wherein said disc discriminating means comprises: a cartridge forenclosing the optical disc; a discrimination hole which is formed on thecartridge and whose opening/closing state differs in correspondence tothe thickness of the disc substrate of the optical disc; and detectingmeans for detecting the opening/closing state of the discrimination holeand for generating a discrimination signal.
 15. An apparatus accordingto claim 13, wherein numerical apertures of at least two or more of theN converging grating couplers differ.
 16. An opticalrecording/reproducing apparatus for recording, reproducing or erasing aninformation signal by converging a light flux onto/from a recordinglayer through a transparent disc substrate, comprising: (a) an opticalhead including: an optical waveguide formed on a substrate, lightemitting means for emitting a waveguide light into said opticalwaveguide, light flux dividing means for dividing the waveguide lightemitted from the light emitting means into N divided waveguide lights, Nbeing greater than or equal to 2, said N converging grating couplers,whose aberrations have respectively been corrected for said N discsubstrates having different thicknesses, each for emitting each of saidN divided waveguide lights to the outside of the optical waveguide andfor allowing the reflected light from the optical disc to enter, and Nphoto detecting means for respectively detecting said reflected lightsfrom the N converging grating couplers and for generating informationsignals; (b) optical head moving means which is arranged below theoptical disc and moves the optical head in the radial direction of theoptical disc; (c) output switching means for selecting and outputtingone of the output signals of said N photo detecting means; (d) discdiscriminating means for discriminating the thickness of the discsubstrate of the loaded optical disc and for generating a discriminationsignal in accordance with the result of the discrimination; and (e)control means for generating a control signal to the output switchingmeans in accordance with the discrimination signal and for selecting thephoto detecting means into which the waveguide light enters from theconverging grating coupler in which the occurrence of the aberration dueto the disc substrate is smallest.
 17. An apparatus according to claim16, wherein said disc discriminating means comprises: a cartridge forenclosing the optical disc; a discrimination hole which is formed on thecartridge and whose opening/closing state differs in correspondence tothe thickness of the disc substrate of the optical disc; and detectingmeans for detecting the opening/closing state of the discrimination holeand for generating a discrimination signal.
 18. An apparatus accordingto claim 16, wherein numerical apertures of at least two or more of theN converging grating couplers differ.
 19. An opticalrecording/reproducing apparatus for recording, reproducing or erasing aninformation signal by converging a light flux onto/from a recordinglayer through a transparent disc substrate, comprising: (a) an opticalhead including: an optical waveguide formed on a substrate, lightemitting means for emitting a waveguide light into said opticalwaveguide, optical path switching means which is arranged on an opticalpath of said waveguide light and switches the propagating direction ofthe waveguide light in N directions in accordance with a control signal,N being greater than or equal to 2, N converging grating couplers, whoseaberrations having respectively been corrected for said N discsubstrates having different thicknesses and which are respectivelyarranged in said N propagating directions which are switched by saidoptical path switching means and emit the waveguide light to the outsideof the optical waveguide and allow the reflected light from the opticaldisc to enter, and photo detecting means for detecting the reflectedlight and generating an information signal; (b) waveguide head movingmeans which is arranged below the optical disc and moves the opticalhead in the radial direction of the optical disc; (c) discdiscriminating means for discriminating the thickness of the discsubstrate of the loaded optical disc and for generating thediscrimination; and (d) control means for generating a control signal tothe optical path switching means in accordance with the discriminationsignal and for switching the propagating direction of the waveguidelight from the light emitting means to the direction of the converginggrating coupler in which the occurrence of the aberration due to thedisc substrate is smallest, wherein the optical head records, reproducesor erases the information signal onto/from the optical disc by the lightflux emitted from the selected converging grating coupler.
 20. Anapparatus according to claim 19, wherein said optical path switchingmeans combines deflecting means for changing the propagating directionof the waveguide light by a deflection angle according to a inputsignal, and wherein said apparatus comprises: tracking error detectingmeans for detecting a tracking error amount of a converged spot whichhas been converged onto the optical disc and for generating a trackingerror signal; and tracking control means for changing the input signalto the deflecting means in accordance with said tracking error signaland for eliminating the tracking error of the converged spot.
 21. Anapparatus according to claim 19, wherein said disc discriminating meanscomprises: a cartridge for enclosing the optical disc; a discriminationhole which is formed on the cartridge and whose opening/closing statediffers in correspondence to the thickness of the disc substrate of theoptical disc; and detecting means for detecting the opening/closingstate of the discrimination hole and for generating a discriminationsignal.
 22. An apparatus according to claim 20, wherein said discdiscrimination means comprises: a cartridge for enclosing the opticaldisc; a discrimination hole which is formed on the cartridge and whoseopening/closing state differs in correspondence to the thickness of thedisc substrate of the optical disc; and detecting means for detectingthe opening/closing state of the discrimination hole and for generatinga discrimination signal.
 23. An apparatus according to claim 19, whereinnumerical apertures of at least two or more of the N converging gratingcouplers differ.
 24. An apparatus according to claim 20, whereinnumerical apertures of at least two or more of the N converging gratingcouplers differ.
 25. An optical recording/reproducing apparatus forrecording, reproducing or erasing an information signal onto/from anyone of N types (where N≧2 ) of optical discs having first layers ofdifferent thicknesses, each type of said optical discs having at leastsaid first layer being transparent and a second layer for storinginformation, by converging a light flux onto said second layer throughsaid first layer of one of said N types of optical discs loaded in saidapparatus said apparatus comprising: a composite converging opticaldevice comprising: (i) a light emitting means for emitting said lightflux; (ii) a converging means for converging said light flux on saidsecond layer of said one of said N optical discs loaded in saidapparatus; and (iii) an optical wave front transforming means disposedin an optical path connecting said light emitting means and saidconverging means for correcting an optical wave front of the light flux,wherein said composite converging optical device (a) performs aberrationcorrection in correspondence with said first layer of said loaded one ofsaid N optical discs, and (b) converges said light flux as a smallerspot diameter D with respect to one of said optical discs having athinner one of said substrates onto said second layer of said loadedoptical disc, wherein said composite converging optical devicedifferently corrects the optical wave front of the light flux incorrespondence with said different thickness of said N optical discs toprovide said aberration correction and said converging of said lightflux, and wherein a thickness of each of said first layers of said Ntypes of optical discs is about 1.2 mm or less.
 26. An opticalrecording/reproducing system comprising: (a) an opticalrecording/reproducing apparatus for recording, reproducing or erasing aninformation signal onto/from any selected one of N types (where N≧2 ) ofoptical discs having first layers of different thicknesses, each type ofsaid optical discs having at least said light layer being transparentand a second layer for storing information, by converging a light fluxon said second layer through said first layer of one of said N types ofoptical discs loaded in said apparatus, said apparatus comprising: acomposite converging optical device, which comprises: (i) a lightemitting means for emitting said light flux; (ii) a converging means forconverging said light flux on said second layer of said loaded one ofsaid N optical discs; and (iii) an optical wave front transforming meansdisposed in an optical path connecting said light emitting means andsaid converging means for correcting an optical wave front of the lightflux; a photo detecting means for detecting reflective light from saidone of said N optical discs, wherein said composite conversing opticaldevice (a) performs aberration correction in correspondence with saidfirst layer of said loaded one of said N optical discs and (b) convergessaid light flux as a spot with a smaller diameter D with respect to oneof said optical discs having a thinner one of said substrates onto saidsecond layer of said loaded optical disc, wherein said compositeconverging optical device differently corrects the optical wave front ofthe light flux in correspondence with said different thickness of said Noptical discs to provide said aberration correction and said convergingof said light flux, and wherein a thickness of each of said transparentsubstrates of said N types of optical discs is about 1.2 mm or less; (b)a signal processing means, responsive to one of (i) a reproductionsignal, corresponding to said information signal, from said photodetecting means and (ii) receipt of recording data, corresponding tosaid information signal, for recording on said disk, for generating anoutput signal corresponding to said information signal for performingone of a reproducing operation and a recording operation on said disks;and (c) a system controlling means coupled to said signal processingmeans for controlling generation of the output signal of said signalprocessing means.
 27. An apparatus according to claim 25 wherein saidcomposite converging optical device has different numerical apertures,and the light flux is converged as a spot with a smaller diameter D byemploying a larger one of said numerical apertures.
 28. An opticalrecording/reproducing apparatus as in claim 25, wherein each of saidfirst layers comprises a transparent substrate.
 29. An opticalrecording/reproducing system as in claim 26, wherein each of said firstlayers comprises a transparent substrate.
 30. An opticalrecording/reproducing system according to claim 26 wherein saidcomposite converging optical device has different numerical apertures,and the light flux is converged as a spot with a smaller diameter D byemploying a larger one of said numerical apertures.
 31. A systemcomprising: (a) an optical recording/reproducing apparatus forrecording, reproducing or erasing an information signal onto/from anyselected one of N types (where N≧2 ) of optical discs having firstlayers of different thicknesses, each type of said optical discs havingat least said light layer being transparent and a second layer forstoring information, by converging a light flux on said second layerthrough said first layer of one of said N types of optical discs loadedin said apparatus, said apparatus comprising: a composite convergingoptical device, which comprises: (i) a light emitting means for emittingsaid light flux; (ii) a converging means for converging said light fluxon said second layer of said loaded one of said N optical discs; and(iii) an optical wave front transforming means disposed in an opticalpath connecting said light emitting means and said converging means forcorrecting an optical wave front of the light flux; a photo detectingmeans for detecting reflective light from said one of said N opticaldiscs, wherein said composite conversing optical device (a) performsaberration correction in correspondence with said first layer of saidloaded one of said N optical discs and (b) converges said light flux asa spot with a smaller diameter D with respect to one of said opticaldiscs having a thinner one of said substrates onto said second layer ofsaid loaded optical disc, wherein said composite converging optical discdifferently corrects the optical wave front of the light flux incorrespondence with said different thickness of said N optical discs toprovide said aberration correction and said converging of said lightflux, and wherein a thickness of each of said transparent substrates ofsaid N types of optical discs is about 1.2 mm or less; (b) a signalprocessing apparatus including: signal processing means, responsive toone of (i) a reproduction signal, corresponding to said informationsignal, from said photo detecting means and (ii) receipt of recordingdata, corresponding to said information signal, for recording on saiddisk, for generating an output signal corresponding to said informationsignal for performing one of a reproducing operation and a recordingoperation on said disks; and system controlling means coupled to saidsignal processing means for controlling generation of the output signalof said signal processing means.
 32. An apparatus according to claim 25,further comprising a disc discrimination means for discriminating thethickness of the first layer of the disc and for generating adiscrimination signal in accordance with a result of the discrimination;and a controlling means for causing the composite converging opticaldevice to converge the light flux as a spot with a particular diameterto minimize aberration due to the first layer in accordance with saiddisc discrimination signal.
 33. A system according to claim 26, whereinsaid optical recording/reproducing apparatus further comprises a discdiscrimination means for discriminating the thickness of the first layerof the disc and for generating a discrimination signal in accordancewith a result of the discrimination; and a controlling means for causingthe composite converging optical device to converge the light flux as aspot with a particular diameter to minimize aberration due to the firstlayer in accordance with said disc discrimination signal.
 34. Anapparatus according to claim 31, further comprising a discdiscrimination means for discriminating the thickness of the first layerof the disc and for generating a discrimination signal in accordancewith a result of the discrimination; and a controlling means for causingthe composite converging optical device to converge the light flux as aspot with a particular diameter to minimize aberration due to the firstlayer in accordance with said disc discrimination signal.
 35. A systemas in claim 31, wherein each of said first layers comprises atransparent substrate.